There exists an active speaker device including an amplifier and a speaker unit outputting sound amplified by the amplifier.
As the speaker unit, for example, there exists the one in which a magnetic circuit including a magnet, a yoke and a coil is used and sound amplified by the amplifier is outputted (refer to, for example, JP-A-2006-229520 (Patent Document 1)).
In such speaker unit, the yoke is arranged at the center of an annular magnet, a coil wound about an outer peripheral surface of a tubular coil bobbin is arranged in a magnetic gap formed between the yoke and the magnet. The coil bobbin can be moved with respect to the yoke in the state where part of the coil bobbin is fitted onto the yoke, a cone is attached to one end of the coil bobbin as well as a cap blocking an internal space of the coil bobbin is attached to one end of the coil bobbin. A flexible damper is also attached to the coil bobbin.
In the above speaker unit, the magnetic circuit is driven based on a sound signal outputted from the amplifier and the cone vibrates with the movement of the coil bobbin, then, sound is outputted. When the coil bobbin is moved, the damper is elastically deformed to thereby suppress excessive movement of the coil bobbin.
In such speaker units, there is a voltage-driven type speaker unit in which sound is outputted by voltage.
The voltage-driven type speaker unit is generally used, however, driving force for outputting sound depends on voltage in the voltage-driven type speaker unit, therefore, linearity between voltage and driving force is broken in many cases, which causes deterioration in sound quality due to the break in linearity.
For example, electric current hardly flows in the coil as a frequency domain becomes higher in the voltage-driven type speaker unit, therefore, output (electromagnetic force) is reduced in high-frequency domains.
In order to compensate the reduction of output in high-frequency domains, the cap with certain rigidity is attached to one end of the coil bobbin and resonance is generated in the vicinity of the cap to amplify vibration, which suppress the reduction of output in high-frequency domains.
In response to the above, there is a current-driven type speaker unit in which sound is outputted by electric current in the speaker units.